In PET scanners pixellated scintillator elements are typically used to convert incident radiation particles to bursts of photons with a wavelength in the UV or visible spectrum. The scintillator elements are typically arranged in a matrix wherein each scintillator element has a base area in the order of 1×1 mm2 to 4×4 mm2. The scintillation events are detected by photosensors coupled to the scintillator elements. State of the art PET scanners use solid-state photosensors, e.g. silicon photomultipliers (SiPMs), typically comprising an array of single photon avalanche diodes (SPADs) being configured to break down responsive to impingement of a photon.
Alternatively, monolithic scintillator elements may be used which consist of a large block of scintillator material. Monolithic scintillators are typically coupled to an array of photosensors configured to localize scintillation events at different scintillator element locations within the monolithic scintillator element.
The size of the scintillator element location that can be identified is a primary factor determining the spatial resolution of the resulting image. Thus, small scintillator element locations are desired to increase the resolution. In the quest for higher resolution solid-state nuclear imaging systems, Anger logic has been used to attain a resolution which is superior to the size of a single photosensor. By coupling the scintillator and the photosensors with a light guide that spreads the emitted scintillation light onto several photosensors and identifying the scintillator element locations with Anger logic, resolution can be improved. Since Anger logic relies on information from neighboring photosensors to identify the scintillator element location, Anger logic becomes inaccurate when information of some of the photosensors is missing, e.g. due to the scintillation event happening during a dead time period of a photosensor or due to individual photosensors being inactive for other reasons.
In order to improve robustness against missing photosensor information, maximum-likelihood methods have been introduced for localizing scintillation events and estimating the energy of those events. Maximum likelihood methods typically include calculating a likelihood for each possible scintillation event location, thereby estimating the energy of the scintillation event. Usually, the location that returns the highest likelihood is assumed to be the originating location of the event. Those maximum-likelihood methods show improved performance compared to Anger logic when measurement information of a single photosensor is missing. However, known maximum-likelihood methods fail if information of multiple photosensors is missing resulting in deteriorated scintillation event localization.
A document “Maximum likelihood based positioning and energy correction for pixellated solid-state PET detectors” by Christoph W. Lerche et al, 2011 IEEE Nuclear Science Symposium and Medical Imaging Conference, Valencia, Spain, 23-29 Oct. 2011, IEEE, Pitscataway, N.J., pp 3610-3613 discloses a detector for gamma-ray detection in the context of a preclinical MR-compatible PET scanner. The detector includes a LYSO crystal array and an array of silicon photomutipliers, together with an intermediate light guide that uses the light sharing principle. A positioning scheme that is based on maximising the likelihood of the scintillation events is disclosed. The method directly gives the index of the active crystal pixel and allows to correct the registered gamma ray energy.